A pool can be filled in 15 hours and drained in 20 hours How

A pool can be filled in 15 hours and drained in 20 hours. How long will it take to fill the pool if both the fill and drain pipes are on? Enter your answer to the nearest hour. (Enter only the numerical value)

Solution

Since the pool can be filled up in 15 hours, therefore, in 1 hour, 1/15^th of the pool can be filled up. Similarly, since the pool can be drained out in 20 hours, hence in 1 hour, 1/20^th of the pool can be drained out. Thus, if both the fill and the drain pipes are on, then in 1 hour (1/15)- (1/20) = (4-3)/60 = 1/60^th of the pool can be filled up. Therefore, the entire pool can be filled up in 60 hours if both the fill and the drain pipes are on. The answer is 60.